Technical Poor idle

[GeorgeDamper]

My 1995 Punto 1.1 8v has recently developed a baffling problem. The idle is very uneven and after driving for 10 minutes the idle speed pulses high / low and then I get a light rattle similar to pinking. The car does not overheat.

It has 45k on the clock and has always been serviced. I have owned Fiats for 30 years and have amassed plenty of spares. I have tried the following spares from my stock:

Coolant temp sensor (blue)
Map sensor
Injector
Lambda
Coils

ICV cleaned
Timing ok
No sign of Head Gasket problem (the previous owner had this done at 35k)
No warning light
Thermostat ok
Plug leads look ok
All service items up to date
Vac pipes ok
No change to MPG
Car garaged every night

Any suggestions welcome

 

[dumbledore]

Had idle problem recently, and in my case it appears that the idle stepper motor was not functioning properly. I swapped it with another from a spare throttle body and it seems to have fixed it for me. Replacing the stepper motor can be a bit tricky though as the stepper motor fixing bolts may not come out easily in which case the whole throttle body will need to come off. Also the motor/ECU has a funny behavior as it needs to calibrate the motor when you first start the engine.

 

[blu73]

i would just swap the whole throttle body over trying to replace any of the sensors - they can be picked up for around £5 from a scrapyard. You may even have a spare lying around in your spares, only a few mins to swap it.

 

[GeorgeDamper]

The scrappy has got one for a mk1 punto 60. Mine is a 55 - do I need to get the ECU as well ?

 

[blu73]

no, one off a P55 or P60 or even a cinq sporting or pre-2001 seicento sporting will do, no need to change anything else.

 

[GeorgeDamper]

Fitted the throttle body and disconnected the battery for 2 hours to reset the ecu. Idling is still poor but that might be the ecu adjusting to the new sensors.

 

[blu73]

well i know you said you checked the vac lines but recheck them. Has it ever had a new fuel filter??? I would compression test it just to be sure as well.

 

[GeorgeDamper]

New fuel filter fitted 2 months ago. I'll get a comp test done.

 

[blu73]

are you sure its fitted the right way around??

 

[dumbledore]

I will check timing and ignition leads.

 

[GeorgeDamper]

Fuel filter correctly fitted
Ignition leads good (I have a spare set)
Compression test good - All 4 cyls around 205

I can get a cheap ECU off a 1.2 from the scrappy - will it fit straight onto my 1.1 and do I need to program it in some way ?

 

[fingers99]

It won't be the ECU.

If you think about it, it's got to be an air leak of some description (or a sleepy IAC, which amounts to the same thing) and you can't get air down a wire.

One way of testing for vacuum leaks (and most ways involve some danger) is to get a propane blowtorch (one without auto ignition!), remove the nozzle, pop some hose on the end of it, and direct it at anywhere a vacuum leak is possible (the gasket at the base of the TB, the vacuum hoses, the servo hose, etc.). If the idle smooths out, you've found your leak.

An ECU is worthless without the code box and key.

205 psi seems a bit on the high side. Carbon build up? After you've sorted the idle, give it a few gallons of 99 octane and cane the ******** off it.

 

[GeorgeDamper]

I've checked for air leaks with carb cleaner and I'm pretty sure there are none.

I connected my strobe light to the lt wires going into the coil packs:
2&3 gave an even regular flash
1&4 gave a more uneven flash which coincided with the rough idle.

1. Does this point to ignition amplifier problems - inside the ECU ?
2. Should the timing mark line up with 1&4 lt wire or both lt wires ?

 

[dumbledore]

As you see two leads uneven, it may be the problem is the ignition coils. There are two coils on the punto, with each one handling two cylinders. One of the coils may be on the way out or indead the the amplifier for that coil is going. If 1&4 is handled by one coil and 2&3 by the other you shoud persue this lead. The coils do die as I know from my own experience.

 

[GeorgeDamper]

The strobe was connected to the wires supplying the coils.

 

[portland_bill]

Swap the two coils (and leads) over and see if the misfire moves to 2&3. If it does, the coil is on its way out, if the misfire stays with 1&4 this would suggest a fault with ECU, wiring, connections, or plugs in those cylinders.

 

[GeorgeDamper]

Fault has stayed with 1&4. I've just opened a can of worms.

 

[dumbledore]

Fault has stayed with 1&4. I've just opened a can of worms.

Probably yes.
The ignition amplifier cannot change the timing. It either does supply the correct current to coils or it does not. So if it is brocken you will not see any strobe light. Again the ECU is responsible for the timing but again as it is all done digitally the timing cannot flactuate. The ECU is barelly following the sensor information to set the timing. So is looking for the crank sensor signal. If the engine is running poorly then the signal from the sensor will become irregular and so will be the timing. So this is a feedback system you are just confirming with the strobe light that the engine runs poorly. So unlikely the problem is the ECU. I would still replace the leads and spark plugs if I were you they are cheap.

 

[GeorgeDamper]

OK thanks

 

[GeorgeDamper]

I never got to the bottom of the problem. It was checked by a specialist diagnostics expert and the local garage.
Last Sunday there was a massive loss of power and black smoke. This was probably caused by the continual slight pinking. The car is now at Albert Looms scrappy in Derby.
The Punto was a great car to drive. It had a proper Fiat funeral with my yellow Panda Dynamic watching the Punto being towed away.